This invention relates to a novel coupling made of a composite of fiber glass reinforced plastic and an integral rubber sleeve for use in interconnecting sections of essentially rigid asbestos-cement pipe for forming a conduit system for carrying fluids under pressure. In the past such systems have in general included lengths of asbestos-cement pipe with precisely turned or machined ends. These ends were interconnected by a coupling made from a relatively large thick walled section of asbestos-cement pipe. This coupling was made by cutting a length of pipe of proper dimensions into a series of relatively short cylinders, and subsequently machining a sealing ring groove into both ends of the interior diameter of each short cylinder of asbestos-cement pipe. While such a coupling system incorporated all the structural, chemical and availability advantages inherent in the use of the asbestos-cement material, forming, in general, an entirely acceptable pressure conduit system, the great number of machining steps needed to form the coupling itself from an asbestos-cement pipe resulted in many drawbacks. Efforts to reduce or eliminate airborne particles of asbestos which result from the various cutting and machining operations necessary to form the coupling have led to concomitant increase in the cost and complexity of these operations. These various machining operations are accomplished under a flood or spray of water to hold the minute particles of asbestos-cement and prevent their loss to the atmosphere. However, increased restrictions on the discharge of this asbestos bearing water effluent makes this solution less economically viable.
The coupling itself tended to be large and massive. Manipulating this massive coupling in the field often resulted in improperly connected pipe Also, the coupling required that the trench be enlarged in the area of the coupling in order to accommodate its greater diameter. Also, the sealing rings, which were positioned in the machined grooves at either end of the coupling, were in constant danger of being displaced during the connection of the spigot ends of the pipes into the coupling.
Attempts in the past to use so-called advanced materials as a substitute for this asbestos-cement coupling have largely met with varying degrees of failure. Each such attempt has incorporated one or a number of the above enumerated disadvantages inherent in the asbestos-cement coupling, or have incorporated into the system certain disadvantages inherent with the substitute material itself. One such inherent disadvantage has been the result of the wide discrepancy between the modulus of elasticity of the asbestos-cement pipe lengths intended to be sealingly connected by the coupling and the lower modulus of elasticity of the material used in the coupling itself. This differential in rigidity between the pipe and the coupling can cause the overall system to fail and leak, especially when there is less than optimum placement of the pipe in the trench. For example, two pipe lengths which are intended to be connected by a somewhat less rigid ring may shift relative to one another when the fill under one such length of pipe is less compacted than the fill under the other length of pipe. Under this condition the weight of one length of pipe is borne by the relatively elastic coupling. The coupling reacts to this stress by deflecting and resulting in a leak between the sealing gasket and one or the other of the pipe lengths. While such deflection and concomitant leaking is less critical and indeed less likely to happen in a nonpressurized conduit system, this deflection and concomitant leaking can have disasterous results in a pressurized system.
Another known coupling was constructed of a fiber glass reinforced epoxy body having a pair of separate gasket seals positioned at each end of the inner diameter. These gaskets were embedded in the fiber glass/epoxy matrix, but required a separate layer of urethane rubber which was sprayed onto the mold mandrel and the individual gaskets to form a liner between the gaskets and to form the bond between the gaskets and the matrix. Also, the cost of epoxy resin and the curable urethane rubber made the coupling too expensive.
U.S. Pat. No. 3,462,175 shows a filament wound coupling which is formed on a cylindrical mandrel with removable rubber inserts which form tapering spiral contours on the inner diameter of the fiber glass/matrix body. But this system requires assembly in the field and the rotary engagement with the pipe ends would be impractical for large diameter pressure pipe.
Another publication of interest is Japanese Pat. No. 61,683 patented Nov. 10, 1924, which shows a connecting coupling having sealing gaskets and an integral connecting liner held in sealing engagement to the ends of pipe sections by steel rings.
Hence, it would be highly advantageous to construct a coupling which would utilize the benefits of the so-called advanced composite materials, such as fiber glass reinforced plastic, permitting the construction of a relatively light, strong coupling, and yet would result in a pressurized conduit system which would tolerate the stresses normal to subterranean installations. Such a system would be desirably a one-piece system to permit easy installation of the coupling arrangement, be inexpensive to manufacture, withstand the rigors of carrying fluid under elevated pressure and be relatively cheap to manufacture.